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Chemical Modeling of Anion Competition on Goethite Using the Constant Capacitance Model¹

ABSTRACT

The constant capacitance model was used to describe phosphate, selenite and silicate adsorption on goethite (-FeOOH) in single anion systems. The model assumes a ligand exchange mechanism and considers the charge on both adsorbate and adsorbent. It is therefore appropriate for describing the adsorption behavior of these specifically adsorbed anions. The computer program FITEQL, a program that fits equilibrium constants to experimental data using an optimization technique, was used to obtain optimal values for the anion surface complexation constants on goethite. The model represented adsorption of these anions well over the entire pH range studied (3–12). The main advantage of the model is its ability to represent changes in anion adsorption occurring with changes in pH. Extension of the model to describing anion competition in mixed systems using the anion surface complexation constants from single anion systems was qualitatively successful. In mixed anion systems the model predictions reproduced the shape of the adsorption isotherms well over the pH range 3 to 12. However, phosphate adsorption was overestimated while adsorption of the competing anion was underestimated.

¹ Contribution from the U. S. Salinity Laboratory, USDA-ARS, 4500 Glenwood Drive, Riverside, CA 92501.

² Soil Scientist.

Received for publication September 24, 1984. Accepted for publication January 22, 1985.

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